Pressure-sensitive recording sheets include a system comprising an upper paper having coated thereon a microcapsule layer containing microcapsules produced by encapsulation of oil droplets of substantially colorless basic leuco dyes dissolved in an appropriate solvent and a lower paper having coated thereon an electron-accepting layer (hereinafter referred to as a "developer layer") containing an electron-accepting compound (hereinafter referred to as a "developer") and, if desired, an intermediate paper having coated on one side thereof a microcapsule layer and on another side thereof a developer layer; a monosheet type comprising a support containing on one side thereof the aforesaid microcapsules and developer; and a sheet comprising a support having contained therein on one of the aforesaid microcapsules and developer and having coated thereon the another.
These pressure-sensitive recording papers are disclosed, e.g. in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,730,457 and 3,418,250 and Japanese Patent Publication No. 49509/73 (corresponding to British Pat. No. 1,396,543).
Heat-sensitive recording sheets using microcapsules are taught in, e.g., Japanese Patent Application No. 99490/84 (corresponding to U.S. patent Ser. No. 735,227 filed on May 17, 1985 and British Application No. 8512303 filed on May 15, 1985). These heat-sensitive recording sheets comprise a support having provided on the same side thereof microcapsules containing a solution of a color former in an organic solvent and a developer capable of reacting with the color former to develop a color. Upon heating, the microcapsule walls become permeable to cause a reaction between the color former and the developer so as to obtain an image. Color formers widely employed are basic leuco dyes, and the developers usable include phenol compounds, organic acids and their metal salts, hydroxybenzoic acid esters, and the like.
The walls of the microcapsules used in the heat-sensitive recording sheet should allow permeation of at least one of the color former and the developer upon application of heat. Examples of high polymeric substances that constitute such capsule walls include polyurethane, polyurea, polyamide, polyester, polycarbonate, urea-formaldehyde resin, melamine resin, polystyrene, styrenemethacrylate copolymer, styrene-acrylate copolymer, and the like.
The microcapsules can advantageously be prepared through polymerization of the reactants which are supplied from the inside of oil droplets. According to this process, capsules that have a uniform size distribution and are suitable for recording materials excellent in preservability can be obtained in a short time.
The above-described encapsulation process and specific examples of compounds used therefor are described in U.S. Pat. Nos. 3,726,804 and 3,796,669.
However, recording sheets having coated thereon these microcapsules containing a solution of a basic leuco dye undergo coloration on their entire surface when exposed to light for a long period of time.
Further, the colored recording sheets have a reduced color-forming property and only provide a low density recorded image. Such a phenomenon scarcely takes place when the basic leuco dye is used as a dispersed solid, but is significant in the cases when the basic leuco dye is dissolved in an organic solvent.
For the purpose of improving the light-fastness of a developed dye image on the lower paper in the pressure-sensitive recording sheet, it is known to incorporate an ultraviolet adsorbent into the microcapsules together with the basic leuco dye solution as disclosed in Japanese Patent Publication No. 14380/69. However, the use of only the ultraviolet adsorbent is insufficient in complete prevention of coloration of the microcapsule sheet.